By shape-shifted what is meant is that the catalyst support component of the present invention changes in external shape and in its internal physical structure without changing in its chemical composition.
Olefin polymerization catalysts comprising an organoaluminum compound and a transition metal compound reacted with a solid magnesium support component are well known in the art. Various magnesium compounds have been employed, including organomagnesium compounds, magnesium halides, magnesium hydrocarbyl oxides, magnesium hydroxide, magnesium hydroxychloride, and the like. It is also well known that the activity of such catalysts and their ability to produce stereoregular olefinic polymers can be enhanced by the addition of an electron donor (Lewis base) independently and/or incorporating it into the supported transition metal component. Such catalysts and processes are described, for example, in U.S. Pat. Nos. 4,414,132; 4,540,679; 4,612,299; and 4,728,705.
Polymer compositions prepared with catalysts derived from magnesium hydrocarbyl oxides, such as described in U.S. Pat. Nos. 4,329,253; 4,393,182; 4,400,302; and 4,414,132 are highly prized due to their advantageous material properties. However, improvements in the resin particle morphology would still be desirable. The catalyst particles tend to be fragile in nature and shatter during polymerization, especially in gas phase polymerization, and produce smaller particles which are irregular in size and shape. Since the polymer produced during polymerization essentially replicates the morphology of the catalyst, the polymers obtained are likewise irregular in size and shape. In addition, some catalysts and the polymers produced with such catalysts contain a relatively high level particles less than 125 .mu.m in diameter, also known as fines, which may coat the walls of the reactor during polymerization and make continuous operation, particularly commercial operation, difficult to sustain.
The morphological disadvantages associated with catalysts derived from some magnesium hydrocarbyl oxides appear to be associated with the external structure of the magnesium starting material as well as with the polymeric, cross-linked internal structure which generates non-optimal internal cohesiveness of the growing catalyst/polymer particle. However, the magnesium hydrocarbyl oxide supports commonly employed in the prior art tend to be fairly intractable both physically and chemically, so that changes in exterior and interior morphology are not readily achieved.
An alternative series of supports, the magnesium hydrocarbyl carbonates, can be prepared from hydrocarbyl oxides. One solution to the problem thus has been to change the morphology and internal structure of the starting magnesium alkoxide composition by conversion to a magnesium hydrocarbyl carbonate composition via treatment of the magnesium alkoxide with carbon dioxide followed by precipitation (U.S. Pat. No. 4,540,679) or by spray drying (U.S. Pat. No. 4,728,705). However, while these compositions result in active polymerization catalysts, they still have some disadvantages. Regardless of the above-described treatment of the magnesium compounds to improve morphological properties, the polymers produced with these catalyst need not be totally identical to polymers produced using magnesium hydrocarbyl oxides. Minute differences in polymer composition are frequently readily detected during commercial extrusion of the polymer into the final product. From the point of view of morphology, these magnesium supports, furthermore, still do not lead to the desired morphology suitable for fluidized processes without further treatment. When the magnesium hydrocarbyl carbonate is precipitated, additional steps such as agglomeration or pre-polymerization of the catalyst are known to be required to achieve the desired particle size of the catalyst and to improve the internal cohesiveness of the resulting agglomerated particles. Spray drying of the magnesium hydrocarbyl carbonate composition is a promising step toward generating larger and more cohesive particles. However, the physical nature of the magnesium hydrocarbyl carbonates (glassy, oily materials) causes formation of unacceptably large amounts of hollow-shelled catalyst and broken fragments derived from such catalyst. On TiCl.sub.4 -treatment of the spray-dried support at elevated temperatures, the sudden release of carbon dioxide causes formation of weak, puffed particles, which in turn readily disintegrate during polymerization. Finally, residual titanium alkoxide trapped in the final particle causes a decrease in catalyst productivity.
Accordingly, a need exists for a catalyst having uniform size and reasonably round shape capable of producing more uniform polymer particles having a high bulk density and a reduced level of fines, and a special need exists for a catalyst with the above properties which also produces a polymer substantially the same as one produced from a magnesium hydrocarbyl oxide support of the prior art.